A comm is short for "communication". There are several ways to comm another person, directly or publicly. Cinder had stated in her book that not many people had used a "direct communication chip" anymore, and the ones that still existed were very slow. Usernames, it seemed in Cress, could be personally edited because Cinder had changed her's to mechanic instead of the, perhaps, default one that might have been given. Prince Kai commed other Earthern officials directly on portscreens, which might have been the traditional, or even only way to comm a person.